Cache/WebGL - Download certain assets at certain times

Hi,

I am aware that using Web GL you can select the cache button which stores the game data to the users computer so the next time the user runs the game it is much faster.

If you were designing a large game, would you be able to tell unity to store certain parts of the game to the cache only when the user needs to download that part of the game?

So instead of the first time the user plays the game the whole things downloads in one go (which would take ages), it downloads only what is necessary then as the user enters the game it begins to download the area they are in, and as they enter another area it will begin to download that area and so forth.

I hope this made sense and was wondering if this was possible.

EDIT: Another question If you were to add an update to your game and release it, then update the game again and remove the previous update, will that update stay in the cache? Is there anyway to keep the cache clean and only have current game content?

Thank you.